The sounding reference symbol (SRS) is used to measure uplink radio channel state information (CSI) between a user equipment (UE) and a base station, e-NodeB (eNB).
In the long-term Evolution (LTE) system, the UE periodically transmits the uplink SRS on the last data symbol of a transmission sub-frame according to such parameters indicated by the base station eNB as transmission bandwidth, starting position of frequency domain, cyclic shift of sequence, period and sub-frame offset, etc. The base station eNB judges the uplink CSI of the UE according to the received SRS, and performs operations of frequency domain selection scheduling, closed-loop power controlling, and so on according to the CSI as obtained.
In the Long-term Evolution-Advanced (LTE-A) system, in order to improve the utilization rate of SRS resources and to enhance flexibility in scheduling of the resources, an aperiodic SRS mechanism is further introduced. To facilitate description, periodically transmitted SRSs are hereinafter referred to as periodic SRSs, while aperiodically transmitted SRSs are referred to as aperiodic SRSs. Currently, the triggering is performed by downlink control information (DCI) for uplink transmission as transmitted from the base station, namely, the user equipment is notified by DCI format 0 and DCI format 4 as to whether to transmit the aperiodic SRS. In this case, the base station eNB can configure for the UE a resource set used to transmit the aperiodic SRS by signaling of a radio resource control (RRC) layer.
Under the following two circumstances in the LTE and the LTE-A systems, in order to transmit the uplink data, the UE firstly needs to perform random access 1) after time-over of an uplink time advance timer (TAT) of the UE occurs, and 2) after a maximum number of transmissions for the UE to transmit a scheduling request (SR) to the base station over a physical uplink control channel (PUCCH) has been reached. After successful random access, the base station eNB may schedule uplink resources for the UE to transmit the uplink data.
In order to effectively schedule uplink physical resources necessary for the UE to transmit the uplink data, it is firstly necessary for the base station eNB to obtain the uplink CSI of the UE. Currently, in order to obtain the uplink CSI of the UE, it is only possible for the base station to perform channel estimation with periodic SRSs transmitted from the UE. However, after occurrence of the aforementioned two circumstances, the UE will release periodic SRSs resources. Accordingly, the currently feasible method is that the base station eNB needs to configure the resources used to transmit the periodic SRS for the UE by signaling of the RRC layer. Then, the UE does not transmit the periodic SRS immediately, but waits until the coming of the transmission time, and transmits the periodic SRS to the base station eNB at preconfigured transmission time. On receipt of the periodic SRS, the base station eNB obtains the uplink CSI from the SRS to thereby schedule resources according to the CSI.
The inventors found the following defects in the prior art during the process of developing the present invention: since it is firstly necessary for the base station to configure resources used to transmit the periodic SRS for the UE by signaling of the RRC layer, while it is necessary for the UE to wait for the coming of the transmission time before transmitting the SRS to the base station eNB, a relatively great time delay occurs when the uplink CSI is obtained by the base station eNB, thereby making it impossible for the base station eNB to rapidly schedule the UE when uplink data need to transmit. Further, under the aforementioned circumstances, there is no effective method of obtaining channel state information by making use of aperiodic SRS in the LTE-A system.